In U.S. Pat. No. 4,519,814 (Demarest) there is disclosed a two-stage glass batch liquefaction process wherein glass batch is preheated by contact with exhaust gas from a liquefying stage to which the preheated glass batch is fed. At the junction of the preheating device and the liquefying vessel the countercurrent flow of exhaust gas and batch material has been found to create some difficulties, some of which are addressed in U.S. Pat. No. 4,496,387 (Heithoff et al.) These difficulties include entrainment of batch material in the exhaust gas stream and premature fusing of the batch material in the preheating device. The last mentioned patent provides substantial relief from these difficulties by vertically segregating the batch stream from the exhaust stream, whereby the batch materials are discharged from the lower side of the preheating vessel, and the exhaust gases are directed by means of a duct to an upper portion of the preheating vessel. Although such an arrangement is advantageous, additional separation between the batch stream and the exhaust would be desirable in order to better avoid entrainment of the batch materials in the exhaust stream and to provide greater freedom in locating the feed point and the exhaust duct on the liquefying vessel.
Constraints on the interconnection of the preheater and the liquefier include the desirability of maintaining the exhaust gas duct as short as possible so as to minimize heat loss, the requirement that a sufficient angle be provided on the batch chute to assure that the batch flows freely by gravity, and the desire to avoid extending a portion of the preheater directly above the liquefier so as to preserve accessibility to the lid region of the liquefier for maintenance. It would be desirable to provide additional space between the exhaust duct and the location at which the batch material is deposited into the liquefying vessel but to do so while maintaining the required angle for batch flow would seemingly require either that portions the preheater would extend above the liquefier or that the preheater and liquefier would have to be spaced farther apart, both vertically and horizontally. Both of these options have drawbacks and therefore are unattractive. In addition to avoiding unnecessary heat loss, minimizing the length of the exhaust duct is also preferred for the sake of avoiding undue enlargement of the relatively cool duct surface upon which entrained and/or vaporized materials can be deposited and accumulate.
U.S. Pat. No. 3,030,094 (Saeman) shows a sloped batch chute and a separate flue connecting a preheating a vessel and a melting vessel. To provide sufficient vertical separation for the batch material to flow by gravity down the chute, the reference relies on rapid rotation of the preheater to cause the batch to cascade through the air space. Such an approach is disadvantageous due to exacerbation of the entrainment problem by the cascading of the batch. Otherwise, a much greater vertical separation between the vessels in the patent would be required, which would undesirably lengthen the exhaust flue. In U.S. Pat. No. 3,508,742 (Minegishi) a lack of vertical separation between the preheater and melter is accommodated by a vibrating ramp and burners that propel batch across the gap. Such an arrangement appears to be difficult to control and susceptible to deterioration in the harsh environment. U.S. Pat. No. 4,113,459 (Mattmuller) shows a vertically oriented preheater which requires a potentially troublesome, moving, mechanical feeder to transfer batch material to the melter. A long, horizontally extending feed chute is shown between the preheater and melter in U.S. Pat. No. 4,185,984, an arrangement that would appear to be subject to plugging and may entail a heat loss problem.